1. Field of the Invention
The present invention relates to shuttle-type image printing devices which print characters and images on a print medium by scanning multiple printheads across the print medium. In particular, the invention provides for improved output from shuttle-type printing devices in which multiple printheads are disposed at a fixed distance from each other and wherein each printhead scans and prints over a divided section of a print medium.
2. Description of the Related Art
Some conventional printing devices use full-line printheads, which are capable of simultaneously printing an entire line of data upon a print medium. Unfortunately, such printheads are quite expensive.
In contrast, serial printing devices operate by scanning a printhead across a print medium. The printhead forms images upon the print medium as it is scanned across. These printheads are required to print only a small amount of data at any one time and are therefore generally less expensive than full-line printheads.
However, one drawback of such serial printing is that high speed printing is difficult to achieve.
Japanese Laid-Open Patent Application No. 50-81437 and U.S. Pat. No. 4,272,771, disclose examples of methods to increase the print speed of serial image printing instruments. According to these documents, the left and right halves of each printed line are printed simultaneously. To achieve this feature, left and right printhead assemblies are provided, both of which are supported by a common carriage mechanism. Accordingly, print speed is approximately doubled over that of serial printing devices. Furthermore, these references suggest that further increases in print speed can be achieved by using more than two printhead assemblies or by printing in both the left and right scanning directions.
Color, high quality and high speed printing have been performed using multiple printheads working in conjunction. While utilizing these configurations, certain relative output levels must be maintained among the multiple printheads. If these relative output levels are not maintained, the color, or gradient, becomes out of balance, or the print density becomes uneven. In general, print quality may be degraded due to incorrect relative output levels.
For example, in a configuration where multiple printheads print in respective divided sections of a print medium, any characteristic differences among the printheads, the ink or the ink ribbons causes mismatches in print density between the divided sections.
FIG. 1A and FIG. 1B illustrate this phenomenon. In FIG. 1A, two printheads, printhead 4A and printhead 4B, have printed within the section designated A and the section designated B, respectively. As shown, printhead 4B produces a more dense output than that of printhead 4A. The Figure illustrates the printing results for three printing duties, 25%, 50% and 100%. The Figure shows that, for each printing duty, the difference in print densities between section A and section B is very noticeable at the border between the two sections.
FIG. 1B illustrates similar printing results utilizing the same printheads while redefining section A and section B so as to add a small overlap between the two sections. Each printhead prints approximately half of the total print data in the overlapped printing area. Hence, the printing density of the overlapped area is greater than that of section A. However, the density is lower than that of section B. Therefore, in the case of FIG. 1B, the density differences are less noticeable than that shown in the above FIG. 1A, but are still obvious at both borders of the overlapped printing area. Accordingly, it is necessary to compensate for differences in print density caused by differences in output characteristics of utilized printheads.
The above problem may be addressed by selecting printheads having the same output characteristics. This approach is not realistic for printing devices in which printheads (or printhead cartridges) can be replaced. Hence, what is needed is a method to compensate for fluctuating output levels of each printhead and to maintain a certain output level balance among multiple printheads.
One important consideration in devising such a method is that, in operation, various printing signals (image data), each having various printing duties, are sent to the multiple printheads of a shuttle-type printing device. The varying duties give rise to varying driving duties of the printheads. The temperature of each printhead will vary accordingly. Furthermore, output characteristics of printing heads are dependent upon their respective temperatures. Hence, when the temperatures of printheads vary during operation, a particular output level relationship of the printheads becomes particularly difficult to maintain.